Use -Wno-undef to compile jsoncpp.
Use -Wno-undef to compile jsoncpp.

This should fix this error:
In file included from ../../api/audio/echo_canceller3_config_json.cc:19:
In file included from ../..\rtc_base/strings/json.h:18:
In file included from ../../third_party/jsoncpp/source/include\json/json.h:9:
In file included from ../../third_party/jsoncpp/source/include\json/autolink.h:9:
../../third_party/jsoncpp/source/include\json/config.h(126,5): error: '__GNUC__' is not defined, evaluates to 0 [-Werror,-Wundef]

Maybe we could also fix it upstream.

Bug: webrtc:10770
Change-Id: I68434c6957189f680625bd2f2b85422965a89785
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145322
Reviewed-by: Artem Titarenko <f2089f8a6cab9eacd475a8c8fad4ad326731ef13@webrtc.org>
Commit-Queue: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#28558}

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